1. Technical Field
The present invention relates generally to aircraft environmental control systems (ECS) and, more specifically, to gas turbine engine compressor air supply to aircraft environmental control systems.
2. Background Information
Environmental control systems (ECS) on board aircraft are used to condition air for the cabin and crew as well as providing cooling for avionics and/or other equipment needing cooling. Some environmental control systems bleed compressed air from the compressor of an aircraft gas turbine engine or of an auxiliary power unit. In a typical closed loop ECS, working fluid passes around a closed loop between a compressor, an intercooler, a turbine driving the compressor and a heat exchanger which extracts heat from the equipment or volume to be cooled.
Bleed air is passed through a precooler to help regulate the temperature and pressure of the bleed air delivered to meet the pneumatic services of the airplane, such as water pressurization, wing and engine anti-ice protection, hydraulic pumps, trim air for cabin warming, and the like. Engine bleed air comes from either a high pressure (HP) or low pressure (LP) engine compressor sections or stages. LP air is used during high power setting operations and HP air is used during descent and other low power setting operations. Control of a HP valve is typically automatic. During low engine thrust operation, the HP valve is open allowing high pressure air to power the system. As thrust is increased, the HP valve automatically closes and an LP check valve opens to supply bleed air allowing low pressure air to power the system.
Engine bleed air is ducted through an air supply precooler. The precooler is a crossflow, air to air, heat exchanger which uses engine fan air as its cooling medium. Fan air is routed to the precooler through the fan air modulating valve which is attached to the bottom of the precooler. The fan air modulating valve regulates the air flow to the precooler based on control air pressure and temperature from fan air pressure and temperature sensors.
Precooled air for the ECS travels through air conditioning packs to provide essentially dry, sterile, and dust free conditioned air to the airplane cabin. This conditioned air is then mixed with a predetermined amount of cabin recirculated air and delivered to the aircraft cabin. Trim air, taken downstream of precooler, may be added to warm the conditioned air to a suitably comfortable level for the aircraft cabin.
Modern turbofan engines with high 5 to 1 or higher bypass ratios have good fuel consumption but produce less compressed air available for ECS. Extracting bleed air for ECS results in a fuel consumption penalty as does the weight of the precooler. A typical precooler is sized for what is referred to in the art as a one engine out heavy hold engine operating condition. This condition is for a fully loaded aircraft, with one engine out, and an engine operating at a relatively low RPM or power level such as might be used in a loitering or approach flight conditions.
This design flight condition may require twice the compressed bleed air a normal operating condition would require due to engine out, thus, not only increasing fuel consumption but also requiring a larger and heavier precooler. With rising fuel costs and the development of more efficient engines, it is highly desirable to decrease the amount of compressed air used by and the weight of ECS precoolers.